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The Earth-like Planet Gliese 581g

Professors Steven Vogt at UC Santa Cruz, and Paul Butler of the Carnegie Institution have just announced the discovery of a new planet orbiting the nearby red dwarf star Gliese 518. The star is located 20 light years from Earth in the constellation Libra. The planet joins five others in this crowded planetary system, and has a mass about three to four times Earth, making it in all likelihood a rocky planet, rather than a gas giant. The planet is tidally locked to its star which means that during its 37 day orbit, it always shows the same face to the star so that one hemisphere is always in daylight while the other is in permanent nighttime.

One of the most important aspects to new planets is whether they are in a distance zone where water can remain a liquid on the planets surface. The Habitable Zone (HZ) location around a star depends on the amount of light energy that the star produces. For the Sun, the HZ extends from about the orbit of Venus to the orbit of Mars. For stars that emit less energy, the HZ will be much closer to the star. Once an astronomer knows what kind of star a planet orbits, they can calculate over what distances the HZ will exist. Problem 1 - What is the pattern that astronomers use to name the discovered planets outside our solar system? Problem 2 - One Astronomical Unit (AU) is the distance between Earth and the Sun (150 million kilometers). Draw a model of the Gliese 581 planetary system with a scale of 0.01 AU per centimeter, and show each planet with a small circle drawn to a scale of 5,000 km/millimeter, based on the data in the table below: Planet Gliese 581 b Gliese 581 c Gliese 581 d Gliese 581 e Gliese 581 f Gliese 581 g Discovery Year 2005 2007 2007 2009 2010 2010 Distance (AU) 0.04 0.07 0.22 0.03 0.76 0.15 Period (days) 5.4 13.0 66.8 3.1 433 36.6 Diameter (km) 50,000 20,000 25,000 15,000 25,000 20,000

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Problem 3 - The Habitable Zone for our solar system extends from 0.8 to 2.0 AU, while for Gliese 581 it extends from about 0.06 to 0.23 because the star shines with nearly 1/100 the amount of light energy as our sun. In the scale model diagram, shade-in the range of distances where the HZ exists for the Gliese 581 planetary system. Why do you think astronomers are excited about Gliese 581g?

Space Math

http://spacemath.gsfc.nasa.gov

Answer Key

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Problem 1 - What is the pattern that astronomers use to name the discovered planets outside our solar system? Answer: According to the order of discovery date. Note: Gliese 581 A is the designation given to the star itself. Problem 2 - Draw a model of the Gliese 581 planetary system with a scale of 0.01 AU per centimeter, and show each planet with a small circle drawn to a scale of 5,000 km/millimeter, based on the data in the table. Answer: The table below gives the dimensions on the scaled diagram. See figure below for an approximate appearance. On the scale of the figure below, Gliese 581f would be located about 32 centimeters to the right of Gliese 581d.

Planet Gliese 581 b Gliese 581 c Gliese 581 d Gliese 581 e Gliese 581 f Gliese 581 g

Discovery Year 2005 2007 2007 2009 2010 2010

Distance (cm) 4 7 22 3 76 15

Period (days) 5.4 13.0 66.8 3.1 433 36.6

Diameter (mm) 10 4 5 3 5 4

Problem 3 - The Habitable Zone for our solar system extends from 0.8 to 2.0 AU, while for Gliese 581 it extends from about 0.06 to 0.23 because the star shines with nearly 1/100 the amount of light energy as our sun. In the scale model, shade-in the range of distances where the HZ exists. Why do you think astronomers are excited about Gliese 581g? Answer: See the bar spanning the given distances. Note that Gliese c, g and d are located in the HZ of Gliese 581. Because Gliese 581 g is located near the center of this zone and is very likely to be warm enough for there to be liquid water, which is an essential ingredient for life. Gliese 581c may be too hot and Gliese 581 d may be too cold.

Space Math

http://spacemath.gsfc.nasa.gov

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